Why is my bathroom sink leaking underneath?
Water beneath a bathroom sink can come from the tap connections, the basin waste, the trap or even splashes running down from above. Finding out when the drip appears is often the quickest way to narrow down the cause.
When does the leak appear?
Dry the cupboard or floor, place a bowl or towel underneath, then watch from a safe position while someone uses the sink. If it leaks as soon as a tap is opened, even with the plug in, a tap tail or water-supply connection may be at fault.
If it only leaks while the basin drains, the waste fitting, overflow connection or trap is more likely. Water that appears after splashing around the rim may be travelling through failed sealant or around the tap base.
Could the trap or waste be leaking?
The trap is the curved fitting beneath the basin. Its joints can drip if a seal is worn, a connection is loose or the pipework has been knocked by stored items. The waste fitting where the plug meets the basin can also lose its seal.
Do not keep tightening plastic fittings: overtightening can distort a washer or crack the trap. A plumber can dismantle the relevant joint, check its alignment and replace the correct seal.
What should I do straight away?
Remove belongings from the wet area and mop up water promptly. If the leak continues when the sink is not being used, close the local isolation valves if they are accessible, you recognise them and they turn without force. Otherwise use the main stop tap and arrange help.
Avoid using the sink until a drain-side leak is repaired. Keep water away from sockets, lighting, electric underfloor heating controls and other electrical equipment; if electrics may be affected, do not touch them and seek urgent professional advice.
When should I call a plumber?
Call a plumber if you cannot see the source, a supply pipe is dripping, a fitting is cracked, the cupboard has swollen, or water has reached the floor or room below. Damp that returns after the visible leak is repaired may mean water is trapped behind the unit or beneath the floor.
Photographs of where the first drop appears and notes about whether it happens during filling, draining or all the time can make diagnosis quicker.
Quick under-sink leak checklist
- Empty the cupboard and dry the area before testing.
- Check whether the leak appears while filling, draining or constantly.
- Look at the tap tails, waste fitting, overflow and trap without dismantling them.
- Catch drips and stop using the sink to limit damage.
- Turn off the appropriate water supply if a live connection is leaking.
- Do not force valves or overtighten plastic fittings.